1. Field of the Invention
The present invention relates to radioactive organic waste treatment methods and systems. Specifically, it relates to methods and systems suitable for treating a radioactive organic waste such as a spent ion exchange resin and filter sludge which contain radionuclides, the radioactive organic waste being generated in nuclear power plants.
2. Description of Related Art
Reactor water cleanup systems and fuel pool cooling cleanup systems of nuclear power plants generate a radioactive organic waste such as filter sludge including a cellulosic filter aid and anion exchange resin. Such radioactive organic waste is hereinafter also referred to as “radioactive spent resin” or simply referred to as “spent resin” or “organic waste”. The radioactive organic waste is stored in a storage tank over a long period of time. The radioactive organic waste is generated steadily with the operation of a nuclear power plant. And the radioactive organic waste is due to be subjected to treatments such as stabilization and volume reduction and to be ultimately disposed of by burial in the ground after the storage.
The ion exchange resins include styrene-divinylbenzene as a base material, are chemically stable, and can be stored safely over a long period of time. The ion exchange resins, however, are hardly decomposable due to their stability and generally require a thermal treatment at a high temperature in order to reduce their volume.
Exemplary methods for treating a radioactive spent ion exchange resin by a thermal decomposition (thermal treatment) can be found as a treatment method using plasma in Japanese Unexamined Patent Application Publication No. 2001-305287 (Patent Document 1); and as a treatment method using microwaves in Japanese Unexamined Patent Application Publication No. Sho 59-46899 (Patent Document 2). The treatment methods in Patent Document 1 and Patent Document 2 respectively promote volume reduction of the spent ion exchange resin.
To solve the problem, there proposed are treatment methods for the volume reduction of the radioactive spent ion exchange resin by another technique than thermal decomposition. Examples of them are as follows.
There are treatment methods of decomposing organic substances in the spent ion exchange resin with hydrogen peroxide. Typically, Japanese Unexamined Patent Application Publication No. Sho 61-270700 (Patent Document 3) describes a radioactive waste treatment method, in which the cellulosic filter sludge is hydrolyzed and liquefied with a cellulolytic enzyme to give a liquid, and the liquid is acted upon by hydrogen peroxide in the presence of an iron ion to oxidize and decompose the organic substances. Ferrous sulfate is used to give the iron ion in the working examples of this document. Japanese Unexamined Patent Application Publication No. Sho 58-161898 (Patent Document 4) discloses a method of bringing a radioactive spent ion exchange resin into contact with hydrogen peroxide in a ferric sulfate aqueous solution and whereby oxidizing and decomposing the ion exchange resin.
Japanese Unexamined Patent Application Publication No. Sho 63-40900 (Patent Document 5) describes a treatment method of the radioactive spent ion exchange resin. By the treatment method, radionuclides contained in a spent ion exchange resin are eluted with a sulfuric acid aqueous solution to remove most of the radioactive substances (radionuclides) from the spent ion exchange resin; the spent ion exchange resin is then converted into an inorganic substance and solidified by an incineration or a chemical decomposition; an eluate containing the radionuclides is incorporated with a divalent iron ion and a base to form ferrite particles; and the radionuclides are taken into the formed ferrite particles and thus separated from the eluate.
Japanese Unexamined Patent Application Publication No. Sho 63-188796 (Patent Document 6) describes a treatment method of a decontamination waste liquid. In the treatment method, a radioactive decontamination waste liquid is treated with a cation exchange resin, and whereby iron and radionuclides in the decontamination waste liquid are scavenged by the cation exchange resin and removed from the waste liquid. The decontamination waste liquid from which the radionuclides have been removed is solidified with cement in a metal drum. Independently, the iron and radionuclides scavenged by the cation exchange resin are eluted out with an organic acid (e.g., oxalic acid or formic acid) to give an eluate containing the eluted iron and radionuclides; and the eluate is given a liquid which converts the iron and radionuclides each into an oxide or hydroxide to be oxidized and decomposed. The oxide or hydroxide is separated from the eluate by a precipitation, and the separated oxide or hydroxide is stored for a radioactive decay. The eluate after the removal of iron and radionuclides becomes a clear water and reused in the nuclear power plant.
Japanese Unexamined Patent Application Publication No. Sho 57-9885 (Patent Document 7) discloses a composition for removing a metal oxide using oxalic acid and hydrazine. The technology is disclosed as not a volume reduction treatment technology, but a chemical cleaning technology relating to such volume reduction treatment.
Japanese Unexamined Patent Application Publication No. 2013-44588 (Patent Document 8) describes a treatment method for a spent resin in a nuclear power plant. The method is described as a treatment method for the volume reduction of filter sludge including a spent ion exchange resin and/or a filter aid. In the method, adsorbed radioactive metal ions are eluted out from the ion exchange resin by an action of oxalic acid (a kind of organic acids); and radionuclides included in crud including an iron oxide are dissolved and removed together with the crud, the crud being deposited on the resin surface. The organic acid (oxalic acid) for use in the treatment is decomposable typically by an oxidizing agent, and this enables the volume reduction of a waste liquid generated as a secondary waste.